I was in the same boat. My FIL has one that I won't be able to inherit as a 12.7... He is keeping it for now as is but I've done the legwork to see what can be done:
The 5" barrels were for a 60-18, but SW does not have any left in their stock. There's a small chance a takeoff barrel might surface, I'm on the lookout.
Ellwood Epps will do a conversion with a bull barrel. But it's Elwood Epps - they are really my last resort for many many reasons.
I have discussed this with Frank Glenn... (He is one of the best SW smiths out there.) He is happy to do the work and make a barrel in whatever profile and length I want - so 4 1/4 " is an option too. Discussed the temporary export with Irunguns and they will do it and subsequently re-register as restricted. Note that Frank says that he is not comfortable with trying to preserve the original barrel - he is afraid of wrecking the alloy frame, so the original barrel has to be cut or he assumes no liability "if the frame bends and [he] cannot straighten it out" and I would trust him on that.
I have since found a different way to keep that gun as is but that's not going to work for everyone, unfortunately...